Layered materials are highly anisotropic, and exist in bulk form as stacks of 2-dimensional (2D) sheets which together form a 3-dimensional (3D) crystal. The bonding in-plane (i.e. within the layer or sheet) is typically comprised of strong chemical bonds, whereas the layers themselves are held together by weaker forces, for example, van der Waals. Layered materials have been studied for over 100 years, but it is only recently, with the isolation of few layer graphene sheets from graphite in 2004 that few-layer, or even single layer, ‘nanosheets’ have been isolated and studied in detail [i]. Since then, increasing numbers of different types of nanosheets have been successfully isolated [ii,iii,iv]. These individual nanosheets can be ˜mm2 in area but are typically ˜nm thick.
The nanosheets can have significantly different, and often enhanced, properties compared with their bulk analogues. For example, the nanosheets have: very high surface areas for gas sensing, catalytic supports and battery/supercapacitor electrodes [iii]; modified electronic structure due to the low dimensionality which, for example, leads to remarkably high electron mobilities in graphene [i], and a direct band gap in MoS2 [ii]; and the nanosheets themselves can be directly imbedded into other materials to form functional composites [iii,iv]. Further to this, exotic physics can arise from the 2-dimensional nature of the material [e.g. see v, vi] and these properties can be tuned by incorporating the nanosheets into nanoscale devices [vi].
There are numerous examples of layered materials other than graphite. Some of the main classes and examples include: transition metal dichalcogenides (TMDC), transition metal monochalcogenides, transition metal trichalcogenides (TMTC), transition metal oxides, metal halides, oxychalcogenides and oxypnictides, oxyhalides of transition metals, trioxides, perovskites, niobates, ruthenates, layered III-VI semiconductors, V-VI layered compounds (e.g. Bi2Te3 and Bi2Se3) [iv] and black phosphorous. While these have been investigated in the 3-dimensional form, in many cases, they have never been isolated as nanosheets. Some of these materials are described in more detail below.
Transition Metal Dichalcogenides: One major class of layered material is the TMDCs [ii]. These are compounds of transition metals (for example Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Tc, Re, Ni, Pd or Pt) with a chalcogen (sulphur, selenium or tellurium) with the formula MX2, where M is the transition metal and X is the chalcogen. In these materials the transition metal is sandwiched between layers of the chalcogen to form an X-M-X stack or sheet. Within the sheets the bonding is covalent, but between the adjacent sheets the bonding is weak, which permits their exfoliation to form nanosheets. These materials have received significant attention because they exhibit a wide variety of electronic properties including metallicity, semiconductivity, superconductivity and charge density waves [ii]. In particular, there has been extensive focus on MoS2 because in the monolayer form, in contrast to the bulk, this material has a direct band gap making it suitable for nanoelectronic applications [ii].
V-VI layered compounds (e.g. Bi2Te3 and Bi2Se3): These materials form in covalently bonded 5-atom stacks/sheets and can be exfoliated into individual nanosheets [iv]. Their main applications are as thermoelectric materials.
Transition metal trichalcogenides (TMTs): These are compounds of transition metals (such as those listed above) with a chalcogen (sulphur, selenium or tellurium) with the formula MX3, where M is the transition metal and X is the chalcogen. Their basic structural elements are prismatic columns of MX6, linked together to build 2D layers [vii]. Metal phosphorous trichalcogenides are transition metal trichalogenides with the formula MPX3, where M is the transition metal, such as Ni or Fe and X is the chalcogen. The latter materials are wide band-gap semiconductors, which suggest optoelectronic applications [iv].
Metal halides: This class includes transition metal halides, which can have formula MX2, MX3, MX4, MX5, and MX6, where M is a metal and X is a halide, for example, CuCl2. These have a wide variety of potentially useful magnetic and electronic properties.
Layered transition metal oxides: This class includes Ti oxides Nb oxides, Mn oxides, and V oxides, which are of interest due to their interesting electronic and dielectric properties with applications in supercapacitors, batteries, as catalysts, dielectric materials and as ferroelectrics [iv].
Trioxides: These include MoS3 and TaO3, which have suggested applications in electrochromic materials and LEDs [iv].
Perovskites and niobites: These include SrRuO4, H2W2O7, and Ba5TaO15, which have suggested applications as ferroelectrics and photochromic materials [iv].
Layered III-VI semiconductors: These include GaX (X=S, Se, Te); InX (X=S, Se, Te), which have useful nonlinear optical properties for optoelectronic applications [iv].
The layered allotrope of phosphorous known as ‘black phosphorous’. This material has recently been exfoliated into nanosheets to form the so-called 2D material ‘phosphorene’. This has a direct band-gap that depends on layer thickness and high carrier mobilities suggesting applications in nanoelectronics [viii].
There is therefore an enormous current global effort investigating the potential of nanosheets for technological applications, or to study the exotic effects that can occur in these materials. However, before the technology can be realised, the first crucial step is to establish protocols to manufacture quantities of high quality nanosheets in a way that is cost-effective and also permits their scalable manipulation into commercial applications.
The two main approaches for producing nanosheets are ‘bottom up’ and ‘top down’. ‘Bottom up’ methods involve growing nanosheets directly onto surfaces, typically via chemical vapour deposition [e.g see ix]. While growth methods are improving, they are difficult to scale as they are limited by the area, and often high cost, of the starting substrate and require high temperatures [ix] which further adds to the expense. Furthermore, the resulting sheets typically have inferior properties (e.g. electron mobility) compared with exfoliated, single crystal sheets [x]. ‘Top down’ methods start with the material in the 3D bulk form, and aim to reduce it to its individual component layers in a process known as exfoliation. Exfoliation methods are typically one of two types. The first is so-called ‘mechanical exfoliation’ where the layers are individually exfoliated, for example, using sticky tape, onto a surface [i]. While this yields high-quality and large-size nanosheets, the process is unpredictable and also difficult to scale. As a result this method is mainly for studying the pristine properties of the nanosheets [e.g. see refs v,vi].
The other exfoliation route is ‘liquid exfoliation’ [see review article iv and references therein]. Here the layered material is exfoliated into nanosheets in the medium of a liquid to form dispersions of nanosheets. The major advantage of this route is that such dispersions can be used to efficiently manipulate the nanosheets into applications in industrially scalable way. For example, from dispersions the nanosheets can be scalably printed into thin films for plastic electronics, or embedded into functional composites [iv].
Current methods for liquid exfoliation typically rely on sonication or chemical reaction to tear apart the nanosheets from their bulk 3D form in the presence of a liquid [iii,iv,xi]. The most commonly implemented process is ultrasonication in certain chosen organic solvents or solvent blends, such as N-methyl-pyrrolidone (NMP) [iii,xi]. This is necessarily followed by centrifugation to remove larger chunks of unexfoliated material in suspension following sonication. Nanosheet dispersions formed via this method are typically an inhomogeneous mixture of mono- and multilayer flakes [iii,xi], have not yet demonstrated complete exfoliation to monolayer units for all materials and can sediment out of dispersion over time [iii]. Further disadvantages are the fact that the sheets can be damaged by the sonication, and that the necessary centrifugation step is difficult to scale industrially.
A modified method of liquid exfoliation involves first intercalating ions between the sheets of the bulk material in order to separate them [iv, xii, xiii, xiv, xv, xvi, xvii]. This increases the layer spacing and is therefore believed to weaken the interlayer adhesion, facilitating exfoliation [iv]. This is typically followed by chemical reaction of the resulting intercalation compound with, for example, water facilitated by (ultra)sonication [xii, xii, xiv, xv, xvi, xvii]. In this process, the water reacts with intercalated lithium ions, forming hydrogen gas between the layers, which blows them apart into nanosheets [xii, xiv, xv, xvi, xvii]. This method of exfoliation has been demonstrated for TMDCs with intercalation via electrochemical intercalation [xvii], intercalation using the salt lithium butyrate [xii, xiv] and intercalation using sodium naphthalenide [xv], and the case of Bi2Te3 compounds via intercalation using lithium-ammonia solution [xvi].
The main benefit of intercalation prior to sonication is that the percentage of monolayers in solution is increased compared to basic liquid exfoliation. However, major problems remain. Damaging sonication is still required and the violent reaction with water can also damage the nanosheets. Furthermore, this process can modify the intrinsic properties of the nanosheets which therefore require further processing in an attempt to recover their pristine properties. For example, in the case of MoS2, ˜50% of the nanosheets deposited from solution lose their desirable semiconducting properties. This is thought to be due to a distortion of the MoS2 nanosheets into a metallic 1T-MoS2 phase as a result of the exfoliation by combined intercalation with lithium and reaction with water [xiv]. The sample has to be subsequently annealed at 300° C. to recover the desired semiconducting 2H-MoS2 phase. However, despite this treatment residual structural disorder still remains [xiv]. Further disadvantages with this method are that dispersions formed have been found to only remain in suspension for several days or weeks [xii], and the nanosheets' surface thought to be hydrated with OH− ions [xviii].
Many other layered materials have been shown to intercalate for example, indium and gallium selenide, MoO3, FeOCl, Heavy Metal Halides, but without demonstration of spontaneous dissolution [iv].
Hence there is a need for a simple yet effective method for producing a thermodynamically stable solution comprising undamaged, unfunctionalised, nanosheets, wherein the method can be scaled up easily to industrial proportions.
To date, the inventors have seen no examples of any of the layered materials listed above being demonstrated to spontaneously dissolve, i.e. dissolve without sonication. In the case of spontaneous dissolution, the individual layers are ‘thermodynamically driven’ into to the solvent. This happens when there is a gain in free energy resulting from the solvation of the charged nanosheets and intercalant ions compared with the combined free energy of the isolated solvent and bulk material. Also, it is well known by the skilled person that TMDCs do not dissolve spontaneously directly in electronic liquids [xix]. Indeed, it has been demonstrated that Li-intercalated MoS2 exfoliated via water reaction actually flocculates when a variety of organic compounds or solvents are added [xx] rather than maintain the suspension. Indeed, despite many studied intercalates of MoS2, none has been shown to dissolve spontaneously without reaction [xx].